In aircraft DME (Distance Measuring Equipment) randomly spaced pulses received from a Tacan ground station are utilized in a known manner to determine the distance between the aircraft carrying the DME and the ground station. Alternating with the randomly spaced pulses are uniformly spaced pulses at a frequency of 1350 Hz, which result in the creation of an audio signal to provide a Morse code indication distinctly identifying the ground station sending the pulses. Equipment must be provided in the DME for distinguishing between random pulses and uniformly spaced pulses. In prior art equipment, an LC circuit has been provided to act as a bandpass filter ideally passing only pulses at the 1350 Hz uniform spacing, which may be passed directly to an audio circuit or alternatively cause the creation of an audio signal.
In an attempt to minimize the bandwidth of the bandpass filter, an inductor and capacitor with rather close tolerances must be chosen. This necessarily results in a high component cost. Further, regardless of how close the inductor and capacitor integrator values are to design specification, some undesired bandwidth in the passed signal is to be expected, resulting in the passage of undesired signals of a frequency near the desired 1350 Hz. Also, the size of at least the inductor is rather large in comparison to the size of the integrated circuits utilized for other circuit components in the DME. Additionally, extra components are required to drive the LC circuit.